The present invention relates to an apparatus for applying optical correlation processing using coherent light to a two dimensional image obtained from an image sensor such as a CCD camera to effect automatic pattern recognition or measurement in the field of optical information processing and optical measurement.
Conventionally, the optical pattern recognition apparatus and correlation processing apparatus generally employ a joint transform correlator. As shown, for example, in FIG. 2, a spatial light modulator of the light addressed type is utilized in such apparatus, as disclosed in Japanese Patent Application Laid-Open Nos.138616/1982, 210316/1982 and 21716/1983. In the FIG. 2 apparatus, a plate 5 has a joint image composed of a reference image, i.e., recognition basis and an input image, i.e., an object of recognition. A laser 1 emits a laser beam which is expanded by a beam expander 2. Thereafter, the expanded laser beam irradiates the joint image on the plate 5 to convert the joint image into a coherent image. The thus obtained coherent image is Fourier transformed by a Fourier transform lens 6. Light intensity distribution of the Fourier transform image is recorded on a spatial light modulator in the form of a liquid crystal light valve 7 which is disposed on a Fourier plane of the lens 6.
Next, an incident light beam divided by a beam splitter 3 is directed through mirrors 14, 15 and a polarizing beam splitter 8 onto the liquid crystal light valve 7 to read out the recorded light intensity distribution of the Fourier transform image. The thus reproduced Fourier transform image is passed through the polarizing beam splitter 8 and is again Fourier transformed by another Fourier transform lens 9 to produce on its Fourier transform plane a correlation image containing correlation peaks indicative of the correlation coefficient between the input image and the reference image. Such peaks are detected by a CCD camera 10.
FIG. 3 shows an example of a joint image composed of an input image and a reference image adjacent to the input image. FIG. 4 shows an example of a pair of correlation peaks detected by the CCD camera 10 and indicative of a correlation coefficient between the reference image and the input image.
However, the conventional apparatus normally treats a pair of a single input image and a single reference image. Therefore, when recognizing a letter of the alphabet, a particular character to be recognized is represented as an input image. In order to examine the correlation relative to all of the alphabetical characters, a reference image is replaced one by one for each character so as to effect sequential correlation processing, thereby consuming considerable time. In order to solve such a problem, concurrent correlation processing could be undertaken between an input image and a multiple of reference images. However, in such a case, the intensity of each correlation peak is seriously weakened due to interference during concurrent correlation processing between the multiple of the reference images and a single input image, while noise increases to undermine weakened correlation peaks and to hinder separation thereof to thereby cause incorrect recognition.
Further, recently it has been suggested, for example, in B. Javidi and C. J. Kuo, Applied optics, 27,663 (1988) that the intensity distribution of the joint Fourier transform image between reference and input images is binarized to form clear and sharp correlation peaks to improve the S/N ratio. However, this prior art construction is still not effective to carry out the concurrent correlation processing without any incorrect recognition.